NetworkManager automatic configuration.

Static address configuration is supported well so far with the exception of the gateway setting not yet separated from the addresses setting. Automatic configuration is a lot more difficult as it depends on DHCP for IPv4 and on a combination of router discovery and DHCP for IPv6.

The main focus will be on IPv6 automatic configuration, as the IPv4 one is working pretty well.

NetworkManager device configuration works in the following sequence:

Gather user configuration and runtime state

Use dynamic configuration protocols to gather information

Perform policy decisions

Commit resulting configuration to the kernel

On the other hand, the Linux kernel autoconfiguration support would:

Use router discovery only, no DHCPv6

Push configuration directly to address lists and routing tables

Notify the userspace daemon about addresses, routes and other parameters

And all of this happens before NetworkManager gets to the policy decision. Modifications of kernel network configuration, especially the address list and the routing table, is a violation of the NetworkManager workflow. It has severe consequences on the correctness and reliability of the whole
configuration process.

Proposed solutions

There is a multitude of ways to solve this problems, some of them are good, some of them are bad. I am going to focus on those that are appealing enough to be discussed.

Solution #1: Fix kernel autoconfiguration and provide respective API

Kernel currently doesn't provide API to do router discovery without immediate reconfiguration. API for addresses, routes nad prefixes would have to be added, only DNS options are currently available. Kernel would basically repackage router advertisements to netlink messages with optional caching.

Solution #2: Use a separate daemon for router discovery

A daemon could be spawned that would listen on a link-local address, recieve router advertisements, parse them, verify them, cache them and provide an event-based API for NetworkManager.